Synthetic seeds are defined as artificially encapsulated somatic embryos, shoot buds, cell aggregates, or any other tissue that can be used for sowing as a seed and that possess the ability to convert into a plant under in vitro or ex vitro conditions and that retain this potential also after storage. In simple words synthetic seed contains an embryo produced by somatic embryogenesis enclosed within an artificial medium that supplies nutrients and is encased in an artificial seed covering.
The technology designed to combine the advantages of clonal propagation with those of seed propagation and storage. Also be as channel for new plant lines produced through biotechnology advances.
The first synthetic seeds were produced by Kitto and Janick in 1982 using carrot
In some of the horticultural crops seeds propagation is not successful due to;
Heterozygosity of seeds particularly in cross pollinated crops
Minute seed size eg; orchids
Presence of reduced endosperm
Some seeds require mycorrhizal fungi association for germination eg: orchids
No seeds are formed
High volume. Large scale propagation method
Maintains genetic uniformity of plants
Direct delivery of propagules to the field, thus eliminating transplants
Lower cost per plant let
Rapid multiplication of plants
Ease of handling while in storage
Easy to transport
Has potential for long term storage without losing viability
Maintains the clonal nature of the resulting plants
Serves as a channel for new plant lines produced through biotechnological advances to be delivered directly to the green house or field
Allows economical mass propagation of elite plant varieties
The somatic embryos foe synthetic seeds are produced in the lab through culturing of somatic cells and treating with different hormones to produce root and shoot. The following are the different steps involved in artificial seeds production;
Establish somatic embryogenesis
Mature somatic embryos
Synchronize and singulate somatic embryos
Mass production of embryos
Encapsulation of matured somatic embryos
Somatic embryos are bipolar structure with both apical and basal meristematic regions which are capable of forming shoot and root, respectively.
Somatic embryogenesis is the development of embryos form vegetative cells with in vitro systems. Specific tissues have a capacity for somatic embryogenesis in cultural systems. This allows the clonal propagation of normally seed-propagated crops analogous to the production of apomictic seedlings. Somatic embryos develop through stages similar to zygotic embryos, however, the final size for the cotyledons are usually reduced and there is no development of endosperm or seed coat.
After pollination, a zygotic embryo of a dicotyledonous species develops through a series of morphological stages termed globular, heart and torpedo. Cotyledons develop and expand as the storage reserves of protein, starch and/or oil are deposited. At some stages before the embryo achieves its maximum weight, it acquires the ability to tolerate drying. Then, the seed's vascular connections to the maternal plant are severed, it stops importing nutrients and it begins to lose water. Seeds of most crop plants can survive drying and can be stored for several years. Once they are hydrated, germination commences culminating in the emergence of a radicle and then the mobilization of the storage reserves by the seedling.
Is the result of a sexual process
Produced as a result of asexual process
involve fusion of male and female gametes
doesn’t involve male and female
Produced form sexual cells
Produced from vegetative cells
contains genetic constituent form both parents
contains genetic constituent form
Genetic recombination takes place
No genetic recombination will take
Contains embryo, endosperm and seed coat
Contains only embryo and endosperm and seed coat are absent
Petiole explants plants are surface sterilized and cultured on SH medium (Schenk and Hildebrandt, 1972) containing 2,4-D, kinetin and many other nutrients. 2,4-D activates the cell cycle of many cells in the petiole - those in the vascular cambium develop into a callus, whereas some sub-epidermal cells develop into a somatic embryo.
The initial somatic embryos, which are only small dense cell clusters at this stage, are embedded in a callus mass of non-differentiated cells.
To liberate these proembryonic structures, and to stimulate the formation of more embryos, the callus is dispersed in a liquid medium to form a suspension culture containing 2,4-D but not kinetin
After 7 days, the suspension is sieved and transferred to solid medium lacking 2,4-D On this medium the embryos develop through morphological stages that appear to be globular, heart and torpedo.
Maturation Phase 1Once the majority of embryos reach the torpedo stage (7-10 days after sieving) they are transferred to an enriched medium containing a high level of sucrose, nitrogen and sulphur to prevent precocious germination and to enable deposition of storage reserves. The embryos rapidly accumulate fresh and dry weight, reaching 1-2 mg dry weight pe
Maturation Phase Ii To induce the acquisition of desiccation tolerance, the somatic embryos are placed on a modified medium containing abscisic acid (ABA) for 3 days. Then they are removed from the medium, washed to remove sugar and other nutrients, and dried.
The standard method of drying is to place the somatic embryos in a sealed chamber over a saturated salt solution designed to give specific relative humidifies. Daily for one week, the embryos are transferred to a progressively lower relative humidity chamber and finally are dried at ambient conditions. At this stage, the embryos have reached approximately 15% moisture and can be stored for a year or more with good viability.
Somatic embryos produced naked embryos without storage materials and protective layer (seed coat). This is very difficult for handling so this demand the encapsulation and coating. The somatic embryos produced are encapsulated using gel agents like agar, alginate, polyco, carboxy methyl cellulose, guar gum, sodium pectate etc. Among these alginate encapsulation was found to be more suitable and practicable. Alginate hydrogel is frequently selected as a matrix for synthetic seed because of its moderate viscosity and low spinnability of solution, low toxicity for somatic embryos and quick gellation, low cost and bio-compatibility characteristics. The use of agar as gel matrix was deliberately avoided as it is considered inferior to alginate with respect to long term storage. Alginate was chosen because it enhances capsule formation and also the rigidity of alginate beads provides better protection to the encased somatic embryos against mechanical injury.
The somatic embryos are mixed with sodium alginate (2 %) and the suspension is dropped into the calcium salts solution (200mM). The principle involved is when sodium alginate dropped into the calcium salt solutions it from round firm beads due to the ion exchange between Na+ in sodium alginate and Ca2+ in calcium slat solutions and sodium alginate form calcium alginate in 20-30 minutes. Since somatic embryos lack seed coat and endosperm the matrix of encapsulation can be added with nutrients and growth regulator, which will serves as an artificial endosperm. This will increase the efficiency of germination and viability of seeds. Addition to these nutrients other useful materials fungicides, pesticides, antibiotics and micro organism can also be incorporated.
Multiplication of non-seed producing plants, ornamental hybrids or polyploids plant
Propagation of male or female sterile plants for hybrid seed production
Germplasm conservation of recalcitrant species
Multiplication of transgenic
Limited production of viable micropropagules that are useful in synthetic seed producer
Asynchrous development of somatic embryos
Improper maturation of somatic embryos that makes them inefficient for germination and conversion in to normal plants
Lack of dormancy and stress tolerance in somatic embryos that limit the storage of synthetic seeds
Somo clonal variations which may alter the genetic constituent of the embryos